A container comprises a wall portion (generally of cylindrical shape), an open neck at an upper end of the wall portion, and a closed bottom at a lower end of the wall portion.
Manufacturing a container by blowing generally comprises inserting a parison (a term designating either a raw injected preform, or an intermediate container obtained by pre-blowing a raw injected preform) into a molding unit having a mold sidewall defining a counter print of the container wall portion and a mold base defining a counter print of the container bottom, said parison having previously been heated at a temperature greater than the glass transition of the material, and injecting within the hot parison a fluid (such as air) under pressure. The blowing may be (and generally is) completed with a stretching of the parison by means of a sliding rod.
As the quantity of material required to manufacture each container continuously decreases over the years for economical and environmental reasons, there is a need for enhancing structural rigidity of the container.
Enhancing structural rigidity of the wall portion is knowingly achieved by means of a series of annular rings the function of which is to minimize the risk of container ovalization (i.e. the container section deforming from a circular shape to an oval shape).
Several techniques are also known to provide structural rigidity of the container bottom. One technique consists of providing the container bottom with stiffening artifacts such as ribs, the function of which is to minimize the risk of collapsing of the bottom under the bending stresses due to the hydrostatic pressure of the content and, possibly, the weight of overlaying containers of a same pallet.
Another technique is to provide an extra stretching of the material of the container bottom to mechanically increase cristallinity (and hence mechanical rigidity) thereof. As disclosed in published PCT application WO 99/52701 (Schmallbach-Lubeca), this is generally achieved by means of a movable mold base slidingly mounted with respect of the mold sidewall, whereby the material of the container is firstly blown beyond the final shape in a retracted position of the mold base, and then embossed to the final shape by the mold base moving to a raised position wherein it completes, together with the mold sidewall, the counter print of the container. The molding unit comprises a cylinder jacket and the mold base has a cylindrical bracket slidingly received within the cylinder jacket.
The mold base is generally moved by means of a linear actuator such as a hydraulic or pneumatic jack, including a cylindrical casing and a rod which is slidingly mounted within the casing and to which the cylinder jacket of the mold base is fixed.
This architecture seams satisfactory but nevertheless has its drawbacks. In particular, a linear actuator of standard quality has a backlash of several tens of degree to several degrees (depending upon the manufacturing precision and wear of the actuator). Moreover, the backlash increases in time with usage. This is why angular precision of base mold guidance generally relies upon the dimensional tolerance between the cylinder bracket of the mold base and the cylinder jacket of the mold sidewall.
In order to enhance precision of base mold guidance, which is required to ensure proper operation of the molding unit and proper shaping of the container bottom, the dimensional tolerance between the cylinder bracket of the mold base and the cylinder jacket of the mold sidewall has to be minimized. However, this leads to an undesired accelerated wear of those components, which hence often need to be replaced.